1. Field of the Invention
The present invention relates generally to optical metrology and, more particularly, to an apparatus and method for rapidly and accurately measuring a surface contour of an object.
2. Related Art
The need to accurately measure object surface contours for dimensional verification adds significantly to unit manufacturing time, as well as subsequent maintenance and repair cost and activities. Typically, the associated processes require the involvement of skilled personnel, particularly where the article has a complex surface configuration. Such objects include, for example, dies, pistons and other objects for which one surface of the object is to be measured, as well as turbine, compressor and power turbine engine blades that require measurement of both sides of the object.
Conventional inspection methods include the use of a plastic template that fits over the blade for comparison of the blade to dimensions indicated on the template. In addition, mechanical calipers have traditionally been used to measure various dimensions on the blade. Such contact or mechanical gauging techniques, however, are susceptible to wear of the gauging device, resulting in a loss of accuracy over time. Moreover, such techniques require operator skill and are time intensive. They are also dependent upon visual inspection, and are generally incapable of providing a direct input to a recording system.
A significant problem with the use of contact or mechanical inspection techniques is the rate of false rejections of serviceable objects such as blades. Oftentimes, blades initially rejected by the use of visual, contact or mechanical gauging procedures have been found to be falsely rejected. Individual blades are costly depending upon the engine size and the function for which the blade is manufactured. With many blades at each stage of an engine, the cost of a high blade reject rate is prohibitive.
To overcome such drawbacks, other conventional approaches for determining the surface contours of objects have been developed. One conventional approach includes the formation of an image of the object, typically acquired by a video camera. The image is digitized and stored in a computer memory as a set of pixels. The computer then analyzes the image, such as by comparing it, pixel by pixel, with a stored reference image. However, there are drawbacks to many such conventional techniques. For example, the processing of a stored image requires a very large number of calculations. Even with high-speed digital computers, processing a stored image requires considerable time, thereby limiting the ability of the system to generate immediate, real-time results. Also, the images are often of poor quality due to the inability to accurately obtain high resolution images of the object.
Coordinate measuring machines (CMMs) have also been used to obtain dimensional information of an object. Typically, a probe is positioned within a three coordinate measurement space to contact an object surface at which time the special position of the probe tip is measured. CMMs, however, must obtain many measurement points to determine a surface contour. For example, to measure the contour of a blade section, a CMM must obtain 20 to 30 measurement points on each side of the blade. As a result, CMMs are impractical to use to measure blades or other objects in a high-volume manufacturing environment. Furthermore, considerable set-up time is required to program CMMs, reducing their availability. More importantly, the accuracy of a CMM degrades when measuring surface contours having a small radius, such as the leading and trailing edges of a blade. Improving the accuracy by increasing the number of measurement points taken at such locations does not significantly compensate for the loss of accuracy and further increases the number of requisite measurement points which must be taken. This further increases the time necessary to obtain an accurate measurement of the dimension of the object. Another drawback to the use of CMMs includes inaccuracies due to the size of the probe tip relative to the smoothness of the measured surface. This further reduces the accuracy with which a given probe tip provides measurement information to the implementing CMM.
More recently, non-contact measurement techniques have been developed to measure surface contours. Typically, single point range sensors using optical triangulation techniques are used to perform non-contact measurements of the dimensions of an object. An illumination source projects a defined area of light onto the surface to be measured. Reflections received from the surface are used to form an image of the light reflected onto a light-sensing detector. As the distance from the sensor to the object surface changes, the position of the reflected image on the detector plane shifts. The lateral shift of position of the image on the detector is used to measure the distance between the sensor and the surface. Such techniques are described in U.S. Pat. No. 4,872,747 to Jalkio et aL. and U.S. Pat. No. 5,362,970 to Pryor et al.
A drawback to such conventional optical triangulation techniques using a single point light source is that in order to obtain high accuracy, the detector must be able to resolve small lateral shifts in the spot position. This generally requires high magnification in the direction of travel of the reflected image. However, the sensor is typically separated from the object being measured by a large stand-off distance. As a result, the sensor has a limited range of motion and must be adjusted in position relative to the blade to retain the object within the measurement range. Furthermore, such conventional systems are typically slow, subjecting them to the above-noted drawbacks.
What is needed, therefore, is a method and apparatus that provides fast and accurate dimensional measurements which can be performed without the need of particularly skilled personnel and without directly contacting the manufactured object.